There are many different tools that can be used for measurement, but the degree of accuracy is dependent upon the tool. For example, a ruler can measure distance with a good degree of accuracy, but for a more precise measurement a caliper should be used. The difference between a brass pressure sensor and a stainless steel pressure sensor is similar if we think about it in these terms:
100 PSIg Stainless Sensor Kit, Stainless Steel Sensor Body, 1/8-inch NPT Male Thread, Includes 100 PSIg Stainless Sensor, Connector, Pins and 1/8-inch NPT to -4 Adapter